1. Field of the Invention
The present invention relates to an inverter circuit and a backlight assembly having the same, capable of lighting a plurality of discharge tubes such as cold cathode fluorescent lamps (“CCFLs”) used for a light source of a liquid crystal display (“LCD”) device.
Although a discharge tube described below mainly represents a CCFL, the present invention is not limited to the CCFL, but adaptable for a system which turns on/off a plurality of discharge tubes in response to an alternating current (“AC”) high voltage. In addition, the discharge tube is not limited to the CCFL.
2. Description of the Related Art
Conventional LCD devices are required to be fabricated having a light weight and compact size while operating with low power consumption. Since the LCD devices are non-emissive devices, a light source must be employed in the LCD device. Typically, a CCFL is used as the light source.
The CCFL is a type of a fluorescent lamp which operates in a regular glow discharge region. An AC high voltage is applied to the CCFL such that the CCFL is illuminated. Since the CCFL is not preheated by a filament, the CCFL has a relatively higher vibration-resistant property, a thinner diameter and a longer life span as compared with those of a hot cathode fluorescent lamp. In addition, since the CCFL is not preheated by a filament, a high voltage must be applied to the CCFL in order to operate.
An inverter circuit is used to generate the AC high voltage used to turn on/off the CCFL.
As shown in FIG. 12, the conventional inverter circuit of the prior art includes first and second inverters 11 and 12, first and second inverter transformers 13 and 14, which belong to a first group, first and second inverter transformers 15 and 16, which belong to a second group and first and second current detection transformers 17 and 18, in order to turn on/off a plurality of CCFLs.
The first and second inverters 11 and 12 control output signals based on a current detection value fedback from the first and second current detection transformers 17 and 18. The first inverter transformer 13, belonging to the first group, has one primary coil 13a connected to an output terminal of the first inverter 11 and two pairs of secondary coils 13b, 13c, 13d and 13e corresponding to the primary coil 13a. The primary coil 13a induces an AC high voltage in the two pairs of secondary coils 13b, 13c, 13d and 13e and the two pairs of secondary coils 13b, 13c, 13d and 13e apply the AC high voltage to the two pairs of the CCFLs 120 connected thereto.
The second inverter transformer 14, belonging to the first group, has one primary coil 14a connected to an output terminal of the first inverter 11 and two pairs of secondary coils 14b, 14c, 14d and 14e corresponding to the primary coil 14a. The primary coil 14a induces an AC high voltage in the two pairs of secondary coils 14b, 14c, 14d and 14e and the two pairs of secondary coils 14b, 14c, 14d and 14e apply the AC high voltage to the two pairs of the CCFLs 120 connected thereto.
The first current detection transformer 17 has one primary coil 17a serially connected to one pair of the secondary coils 14d and 14e of the second inverter transformer 14, belonging to the first group. The first current detection transformer 17 has a secondary coil 17b which is connected between the first inverter 11 and a ground GND so as to create a current detection value and provide the current detection value to the first inverter 11.
The first inverter transformer 15, belonging to the second group, has one primary coil 15a connected to an output terminal of the second inverter 12 and two pairs of secondary coils 15b, 15c, 15d and 15e corresponding to the first primary coil 15a. The primary coil 15a induces an AC high voltage in the two pairs of secondary coils 15b, 15c, 15d and 15e and the two pairs of secondary coils 15b, 15c, 15d and 15e apply the AC high voltage to the two pairs of CCFLs 120 connected thereto.
The second inverter transformer 16, belonging to the second group, has one primary coil 16a connected to the output terminal of the second inverter 12 and two pairs of secondary coils 16b, 16c, 16d and 16e corresponding to the first primary coil 16a. The primary coil 16a induces an AC high voltage in the two pairs of secondary coils 16b, 16c, 16d and 16e and the two pairs of secondary coils 16b, 16c, 16d and 16e apply the AC high voltage to the two pairs of CCFLs 120 connected thereto.
The second current detection transformer 18 has a primary coil 18a serially connected to one pair of the secondary coils 16d and 16e of the second inverter transformer 16, belonging to the second group. In addition, the second current detection transformer 18 has a secondary coil 18b connected between the second inverter 12 and the ground GND so as to create a current detection value and provide the current detection value to the second inverter 12.
Further, a tube-current equilibrium circuit having the conventional inverter circuit of the prior art is disclosed in Japanese Patent Unexamined Publication No. 2005-317253. Referring to Japanese Patent Unexamined Publication No. 2005-317253, the tube-current equilibrium circuit includes a plurality of discharge tubes connected to each other in parallel to serve as loads of the inverter circuit, and balance coils which equalize tube current of the discharge tubes. The discharge tube is a CCFL representing an equivalent resistance value of 50 KΩ or more when current flows therethrough. The balance coil includes two coils having the same number of windings, and a self resonant frequency of the balance coil is 1.5 times an operational frequency of an inverter transformer of the inverter circuit.
In the conventional inverter circuit of the prior art, the current flowing through a plurality of CCFLs is not uniform and thus luminance mura occurs due to an impedance mismatching between the CCFLs, mismatching between inverter transformers and a difference in coil ratios of primary coils to secondary coils of the inverter transformers, even if the current of one CCFL is fedback to the inverter.
Further, in the conventional inverter circuit of the prior art, since the CCFLs generally require a relatively high voltage (starting voltage) to operate the CCFLs (discharge tube), if the CCFLs (discharge tubes) connected to secondary coils of inverter transformers coupled with the first and second current detection transformers 17 and 18 are turned on, and some of the CCFLs (discharge tubes) connected to inverters coupled with the above inverter transformers are not turned on, a starting current of the inverter transformer is in a stable state due to the feedback of current. Accordingly, the voltage of the second coils, which are not driven, does not reach the starting voltage, thereby causing some of the CCFLs not to be turned on.
In addition, in the conventional inverter circuit of the prior art, since current detection interconnections are necessary with the secondary coils of the inverter transformer, designs of the secondary coils of the inverter transformer are different from each other, thus disturbing current uniformity in the secondary coils of the inverter transformer.